There is a 70% chance that the 2025-2029 period will be more than 1.5C hotter than pre-industrial times, World Meteorological Organization (WMO) forecasters predicted in a report on Wednesday.

The 1.5C threshold is symbolically important, as all governments agreed in the 2015 Paris climate agreement to try to limit global warming to that level. Since then, diplomats presiding over climate talks have described the temperature goal as the world’s “North Star” and pledged to “keep 1.5 alive”.

The WMO’s forecasters argue that, if average global temperatures in the years from 2025-2029 are more than 1.5C hotter than the pre-industrial period of 1850-1900, this does not necessarily mean that the Paris Agreement goal would be breached. They and scientists working with the Intergovernmental Panel on Climate Change (IPCC) say temperature rises should be measured in 20-year not five-year averages.

Heading for 1.5C

Nonetheless, Adam Scaife, a British physicist who worked on the latest WMO “annual to decadal climate prediction” update, told reporters that keeping warming below 1.5C – even over a longer time-frame – “would require a fortuitous intervention of natural climate variability”.

This could include, he said, a La Niña weather phenomenon or negative Arctic Oscillation leading to Eurasian winter cooling. But “it’s very unlikely that natural variability is going to come to our aid in that particular manner,” he added.

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Scaife’s colleague Leon Hermanson, from the British Met Office, added that a volcanic eruption “would change the forecast quite a lot”. Volcanic eruptions release sulphur dioxide which reflects sunlight from the earth, causing a large but temporary drop in global temperatures.

In the Paris Agreement, all governments signed up to limiting global temperature rise to “well below 2C above pre-industrial levels” and to “pursuing efforts to limit the temperature increase to 1.5C above pre-industrial levels, recognizing that this would signficantly reduce the risks and impacts of climate change”.

Governments never agreed how to define a 1.5C rise in global temperatures, but the influential IPCC scientists said it should be measured as an average over a 20-year period.

Twenty-year average

To assess this with real-world observations would mean waiting ten years from any particular year to gather enough data to know if the average had surpassed 1.5C in that year. So instead the WMO’s forecasters estimate the 20-year average by using observations for the past ten years combined with predictions for the next ten years.

Using this way of calculating the 20-year average for 2024, they found that last year the world was 1.44C hotter than pre-industrial levels – even though 2024 taken alone was 1.55C above pre-industrial levels. “We are still shy of 1.5C in the global average,” Scaife said.

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The WMO report says there is an 80% chance that at least one year between 2025 and 2029 will be warmer than the hottest year on record – currently 2024 – and there is an 86% chance that at least one year will be more than 1.5C above the pre-industrial period.

The forecasters also estimate there is a 1% chance that one year between 2025 and 2029 will be more than 2C above pre-industrial levels. Scaife called this a “shocking possibility”, which had gone from “effectively impossible just a few years ago” to now just “exceptionally unlikely”.

Speaking to reporters online from a “sunny Geneva”, the WMO’s director of climate services Chris Hewitt said it’s “tempting to get fixated” on whether the 1.5C limit has been breached but “every fraction of a degree matters – it’s really important to keep the warming as low as possible”.

He said that the Paris Agreement has reduced the amount by which global temperatures are predicted to rise, and the COP30 climate talks in Brazil in November are “an opportunity for the world to come together” and for “the decision makers and policy makers to take climate action”.

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Scientists predict global warming of more than 1.5C for 2025-2029 period